devel/ccls: Fix build with multiple llvm* packages installed

Apparently it is possible for the build to pick multiple LLVM
versions if LLVM_DIR is not explicitly set similar to Clang_DIR.

$ grep llvm $(make -V CONFIGURE_WRKSRC)/CMakeCache.txt
Clang_DIR:PATH=/usr/local/llvm80/lib/cmake/clang
LLVM_DIR:PATH=/usr/local/llvm50/lib/cmake/llvm

PR: 237089
Submitted by: fullermd@over-yonder.net